We have recently shown that at low ionic strength ( Mu = 0.02 M), 5 C, in the presence of ATP and absence of Ca++, relaxed skinned rabbit psoas fibers show substantial stiffness when rapidly stretched (v greater than 10tothe4 nm/half-sarcomere/s). This stiffness is approximately proportional to the amount of overlap between thick and thin filaments which suggests that it is due to cross-bridges bound to actin. We have characterized this cross-bridge behavior by examining the dependence of the stiffness due to cross-bridges on velocity of stretch. When stiffness in low ionic strength ATP relaxing solution was plotted versus velocity of stretch over several orders of magnitude, the relationship was highly non-linear; for a given change in velocity, there was less increase in stiffness at high velocity than at low velocity. To determine whether the stiffness-velocity of stretch relationship is similar when ligands other than ATP are bound to the cross-bridge, we measured fiber stiffness in the presence of 4 mM PPi, 6 mM MgCl2, 1 mM EGTA, 20 mM imidazole and 80 mM KCl. (In this solution, the in vitro binding constant of myosin subfragment-1 to actin is about the same as in ATP at Mu = 0.02 M). In this PPi solution, stiffness was again a highly non-linear function of velocity. These stiffness data are best explained by assuming that, both in PPi and in relaxing ATP solutions, cross-bridges are attached in a relatively rapid equilibrium between attached and detached states. From consideration of the velocity of stretch necessary to attain a given stiffness, the attachment/detachment rates in relaxing ATP solution appear to be about 10tothe3 faster than in the PPi solution. This suggests that the cross-bridge attachment rate in muscle is not diffusion limited. Furthermore, the finding of significant cross-bridge attachment rates in a relaxed muscle suggests that a muscle is kept relaxed by blocking kinetic step subsequent to attachment, and not by blocking attachment as required by the classic steric blocking model.